As shown in FIG. 2, a conventional, prior art, conveyer unit includes a plurality of rollers 101, 102 for conveying articles between a pair of side rails 100, 100 disposed in parallel. The rollers include a plurality of freely rotatable idle rollers 101 and drive rollers 102 to cause articles to be compulsorily conveyed. Each drive roller 102 is rotatable via direct connection with a motor incorporated therein (hereinafter called motor-incorporated roller) or via linkage with an external drive unit such as a motor. A sensor 103 secured to a side rail 100 detects whether articles are actually being conveyed on rollers 101 and 102. If the sensor 103 detects that preceding articles remain in a halt, the drive rollers 102 come to a halt to discontinue conveying of the following articles so that the articles can be prevented from colliding with each other. Accordingly, in normal cases, the conventional conveyer unit causes drive rollers 102 to remain in operating condition even when no article is conveyed, thus resulting in the waste of energy and rapid abrasion of mechanical parts.
Further, in the case of so-called concentrative control systems which use a single controller to control a conveyer system comprising a plurality of conveyer units disposed in series, a large number of cables are required. This requires much time and labor costs for installation, thereby making it quite difficult to extend or contract or change lines of the conveyer system whenever adding or removing or changing conveyer units.
To deal with this disadvantage, a conveyer system designed for effectively activating and halting operation of the drive rollers, and a control system for controlling said conveyer system, has been disclosed in the U.S. Pat. No. 5,285,887. In this conveyer system, interspace between a pair of side rails disposed in parallel is divided into a plurality of zones in the direction of conveying articles. Each zone is provided with drive rollers independently driven in the forward direction and in the reverse direction, a sensor for detecting presence or absence of conveyed articles, and a controller for activating the drive rollers when articles are detected by the sensor. This is a so-called dispersive control system.
In the above dispersive control system, the drive rollers usually remain in a halt. A drive roller in the system is activated, for example, when the sensor in the zone with which the drive roller is provided detects any article and the sensor in the adjacent downstream-side zone detects no article. Accordingly, the control system provided for the above conveyer system can be activated with efficiency of energy.
Further, the Laid-Open Japanese Patent Publication No. HEI-7-172549 has disclosed a conveyer unit via a dispersive control system which facilitates easy change of layout, removal, and reassembly of a conveyer system. The above conveyer unit incorporates rollers for conveying articles, a driver for driving said rollers in the forward and reverse directions, a controller for controlling said driver, and two sensors for detecting presence or absence of articles on the rollers. The controller incorporates a CPU (control means) for executing a plurality of convey-control programs by way of selecting any of memories and the convey-control programs. It is possible to select a proper convey-control program by way of setting parameters. Accordingly, even when the conveyer unit is installed at the downstream end, or at an intermediate portion, the layout can easily be changed since a proper operation can be executed merely by changing parameters.
Further, the Laid-Open Japanese Patent Publication No. HEI-7-206132 has disclosed such a conveyer capable of computing the dimension along the conveying direction of a conveyed article according to the sensor-ON time and the conveying speed of the conveying means. The system uses the gained dimensional data and the conveying speed to sequentially drive respective conveying means (rollers) located at article-conveying positions.
The above-referred conveyer disclosed in U.S. Pat. No. 5,285,887 is divided into a plurality of zones to effect driving and halt of the conveyer per zone. Accordingly, this provides an advantage whenever conveying an article that properly fits the size of the zone, whereas it provides a disadvantage whenever conveying an article being significantly greater than the size of the zone. As shown in FIG. 3, for example, assume that the conveyer is split into zones a through f, if a conveyed article 2 having a dimension extending itself from zones a through e is placed on the conveyer, it generates difficulty to properly convey the article 2. Concretely, zone e can be driven because no article is present in the downstream-side zone f, whereas the other zones a, b, c, and d, respectively, hold on the halted condition because the sensors at the downstream-side zones have detected the presence of an article. Because of this, when a conveyed article extending itself across a plurality of zones is placed on the conveyer disclosed in U.S. Pat. No. 5,285,887, only the foremost zone is driven, whereas the remaining zones cannot be driven. Accordingly, when operating the conveyer disclosed in U.S. Pat. No. 5,285,887, it is difficult to convey a sizable article thereon. Because of this disadvantage, whenever changing size of articles, it is necessary to replace the conveyer itself.
Further, there has been a problem in this conveyer because the conveyer could not properly accommodate itself to convey articles having a variety of sizes in mixture.
Additionally, when operating the above conveyer disclosed in U.S. Pat. No. 5,285,887, discontent is generated because of slow conveying speed even when conveying an article that properly fits the size of the above zones. As shown in FIG. 4, for example, assume that articles 3 are individually placed on the above-referred zones a through e. In this case, the foremost zone e must initially be driven to cause the foremost article 3 to be shifted to leave zone e so that the second article 3 in zone d can be shifted to zone e. After causing the foremost article 3 to leave the following zone f, said zone e carrying the second article 3 is driven. Thus, whenever operating the conveyer disclosed in U.S. Pat. No. 5,285,887, overall conveying speed is quite slow because articles are conveyed via an individual zone with each zone being frequently and repeatedly driven and brought to a halt. Further, when operating the above conveyer disclosed in U.S. Pat. No. 5,285,887, each conveyed article 3 is gradually separated in the course of conveying operation, thereby causing increased time and labor costs associated with the reception work at the conveying destination.
Likewise, the conveyer unit disclosed in the Laid-Open Japanese Patent Publication No. HEI-7-172549cannot control individual rollers that make up a conveyer unit, thereby resulting in difficulty in driving and halting the rollers in correspondence with a variety of forms of articles.
On the other hand, the conveyer disclosed in the Laid-Open Japanese Patent Publication No. HEI-7-206132 computes the dimensions of articles along the conveying direction based sensor-ON time and conveying speed of conveying means. Nevertheless, since the conveying speed used for the above computation is estimated from rotating speed of rollers, error could be generated between the estimated speed and the actual conveying speed to cause error to be generated in the computed dimension of articles. Because of this, discrepancy could occur between those conveying means (i.e., rollers present at positions for conveying articles and those conveying means (rollers) actually being driven.